1. Field of the Invention
This invention relates to a foil bearing. More particularly, this invention concerns a foil bearing wherein undesirable metal to metal contact between the metal foil and the rotary shaft of the foil bearing, during the high speed rotation of the rotary shaft is prevented from occurring. Even more particularly, the present invention concerns foil bearings having a pair of relief portions on both ends of the metal foil in the direction of the axis of the rotary shaft, or on the positions of the rotary shaft corresponding to said ends of the metal foil or on the two opposite portions on the outer surface of the rotary shaft each starting from the point further in than the position corresponding to each end of the metal foil up to the outer axial end of the rotary shaft.
2. Prior Art
Heretofore, foil bearings as radial gas-containing bearings have been known. This type of foil bearing is constructed in such a manner that the rotary shaft of the bearing is wrapped by a flexible metal foil of aluminum or stainless steel, etc., such that the pressure of a gas layer formed between the thin foil and the rotary shaft can support the rotary shaft.
It is an indispensable requisite in this type of foil bearing to maintain the bearing clearance between the metal foil and the rotary shaft constant, even when the rotary shaft rotates at a very high speed, to ensure complete lack of contact between said two mating parts.
However, it is very difficult, in the actual fabrication process, to finish each of the metal foils with perfectly uniform smoothness over the entirety of their bearing surface. Accordingly, some defects such as burr, warping or waved surfaces are liable to be caused on both axial ends, or at the margins of the cut metal foil.
Assembled foil bearings using metal foils having such defects are liable to cause undesirable metal-to-metal contact during high speed rotation between the metal foil and the rotary shaft. Thus, it becomes difficult or impossible to assure perfect non-contact over the entire surface of the metal foil.
As a pressure characteristic of the gas layer in the bearing clearance defined between the metal foil and the rotary shaft, the pressure distribution curve along the axis of the shaft takes the form of a relatively smooth arcuate shape when only a small extent of bearing load is applied to a foil bearing having large bearing clearance. Because of the small load there will not be an appreciable amount of excessive pressure even at the axially middle portion of the bearing.
On the other hand, when a large extent of bearing load is applied to a foil bearing having a small bearing clearance, the pressure distribution curve of the bearing will take an arcuate shape having sharp curvature resulting in excessive pressure at the middle part of the bearing, along the axis of the rotary shaft.
As explained above, if a large extent of a load is applied to a foil bearing having small clearance, when the bearing is operated at high speed of rotation, the middle portion of the metal foil of the bearing along the bearing axis is subjected to a large extent of gas layer pressure and will expand and deform radially outwardly, resulting in contacting of the metal foil at both axial ends with the rotary shaft. This invention aims to obviate the problems as explained above.